1. Field of the Invention
The present invention relates to a wire dot printer head and a wire dot printer.
2. Discussion of the Background
There has been known a wire dot printer head wherein an armature with a printing wire connected thereto is pivoted between a printing position and a stand-by position, and when the armature is pivoted to the printing position, a tip of the wire is brought into collision with a printing medium such as a paper to effect printing. In a certain wire dot printer head of this type, there has been proposed a device wherein a magnetic flux is produced by a coil around the armature to be pivoted for forming a magnetic circuit that causes the armature to be attracted from a stand-by position to a printing position to effect printing (see Japanese Published Unexamined Patent Application No. 2001-219586).
As shown in FIG. 5, in the wire dot printer head disclosed in the patent document 1, an armature 101 holding a printing wire is provided at the position that is a printing position and opposite to a core 103 having a contact face 102 that comes in contact with the armature 101. This armature 101 has a contact face 104 that comes in contact with the core 103 at its printing position and a pivot shaft 105. The armature 101 is provided pivotably with the pivot shaft 105 as a center. This pivot shaft 105 is mounted on a yoke that is for forming a magnetic circuit, and is supported by the yoke 106 and a side yoke (not shown) (in more detail, see FIG. 1 in Japanese Published Unexamined Patent Application No. 2001-219586).
Further, provided on the side yoke is an elastic spacer for fixing the position of the pivot shaft 105 of the armature 101 with elastic force (in more detail, see FIGS. 3 and 4 in Japanese Published Unexamined Patent Application No. 2001-219586). This prevents the shake of the pivot shaft 105 during the printing operation, thereby enhancing durability of the yoke 106 and the side yoke.
However, the armature 101 pivots as much as 2500 times per second between the printing position and the stand-by position with a recent increased printing speed, so that vigorous vibration occurs during the printing operation. In the wire dot printer head disclosed in the patent document 1, it is difficult to restrain the movement of the pivot shaft 105 of the armature 101 due to the vigorous vibration, particularly it is difficult to prevent the movement of the pivot shaft 105 of the armature 101 in the radius direction of the yoke 106 (in the rightward direction in FIG. 5). Therefore, the pivot shaft 105 slightly moves, thereby wearing out the surface of the yoke 106 and the side yoke. Further, the pivotal movement of the armature 101 is not stabilized Moreover, even in case where the pivot shaft 105 is held by another supporting member other than the yoke 106, the pivot shaft 105 moves in the radius direction of the yoke 106, thereby wearing out the surface of the supporting member.
On the other hand, supposing that, at an intersection point K between a virtual circle A wherein the shortest distance from the center of the pivot shaft 105 to the center of the contact face 104 is defined as a radius with the center of the pivot shaft 105 of the armature 101 as a center and the contact face 104, attraction force acted on the contact face 104 by the core 103 is defined as F and the angle made by this attraction force F and the tangential direction is defined as θ as shown in FIG. 5, force of the attraction force F in the tangential direction, i.e., component of force in the rotating direction becomes Fcosθ. This component of force in the rotating direction Fcosθ becomes pivotal force of the armature 101. In this case, the armature 101 in the patent document 1 has the angle θ of about 45 degrees, as shown in FIG. 5, that is great, so that the attraction force F and the component of force in the rotating direction (pivotal force) Fcosθ are greatly different from each other. Specifically, the attraction force F is not effectively converted into the pivotal force of the armature 101. This cannot provide pivotal force required for high-speed printing.
Further, in the armature 101 in the patent document 1, force acted in the radius direction of the yoke 106 (in the rightward direction in FIG. 5) is great in the component of force in the rotating direction Fcosθ, so that the pivot shaft 105 moves in that direction. This provides the non-stabilized pivotal movement of the armature 101. In particular, the surface of the yoke 106 and the side yoke made of a magnetic material is worn out due to the movement of the pivot shaft 105 of the armature 101. It is possible to prevent the wearout of the yoke 106 and the side yoke by providing a hardening process such as a nitriding on the surface of the yoke 106 or the surface of the side yoke. In this case, magnetic characteristic of the magnetic circuit is reduced. Therefore, pivotal force required for high-speed printing cannot be obtained.